1. Field of the Invention
The present invention relates to a piezoelectric motor constructed as to utilize a variation of the strain of a piezoelectric element and an electrostrictive element to provide a rotary output at a shaft thereof, wherein, for example, the piezoelectric element is composed of plumbum zirconate titanate (PZT) and the electrostrictive element is composed of plumbum magnesium-niobate (PMN).
2. Prior Art
Piezoelectric motors, for example a motor, as shown in FIG. 8 hereof and as disclosed in Japanese Provisional Patent Publication No. 60 (1985)-160384, are known in the art.
Around a rotor 52 of such a piezoelectric motor there are provided plural electrostrictive elements 53a to 53d of which the strain directions are directed toward the center of a rotary shaft 55 of the motor. These electrostrictive elements 53a 53d are supplied to respectively different phase voltages Ea to Ed, as shown in FIG. 5, and the rotor 52 is driven in a circular motion without rotating on its axis by the strains of the respective electrostrictive elements 53a to 53d. The rotary shaft 55 engages loosely with a circular hole 54 formed in the central portion of the rotor 52 and is supported rotatably by a motor case 51. Thus the rotary shaft 55 is driven rotatively with its exterior surface kept in contact with the surrounding interior surface of the circular hole 54, by the circular motion of the rotor 52.
In this case, a clearance .DELTA. between the radius R of the circular hole 54 of the rotor 52 and the radius r of the rotor shaft 55 is equal to 2(R-r). Since the rotor 52 is adapted not to rotate on its axis and further the contacting surfaces of both the circular hole 54 and the rotary shaft 55 are adapted to slipping, the rotary shaft 55 is rotated at a speed reduction ratio corresponding to 2.pi.(R-r)/2.pi.r=(R-r)/r from the circumferential difference 2.pi.(R-r) between the interior circumferential 2.pi.R of the circular hole 54 and the exterior circumferential 2.pi.r of the rotary shaft 55. Therefore, the speed reduction ratio per one circular motion of the rotor 52 becomes .DELTA./2r, and this is in proportion to the clearance .DELTA..
This type of piezoelectric motor is simple in construction, can be small-sized and is able to attain a low rotational speed without requiring any reduction gears. Furthermore, since it has no magnetic coil, such as that used in a general motor, it is useful as a driving source of a driving apparatus that should not be affected by a magnetic field.
However, in the above-mentioned prior art piezoelectric motor, since the rotary shaft 55 engages loosely with the circular hole 54 of the rotor 52, a certain amount of clearance .DELTA. is required therebetween in order that the rotary shaft rotates with its exterior surface kept in contact to the surrounding interior surface of the circular hole 54. For example, in the case of the radius of the rotary shaft being 10 mm, since at least 40 .mu.m is necessary for the clearance .DELTA., a speed reduction ratio of approximately 1/500 (i.e., 40 .mu.m/2.times.10 mm) is attained.
Accordingly, when a smaller speed reduction ratio (for example, of the order of 1/10000) is required, the clearance .DELTA. must be made correspondingly smaller sufficiently than 40 .mu.m. But it is almost impossible to make the circular hole 54 of the rotor 52 and the rotary shaft 55 to satisfy such a condition. Hence, a very small speed reduction ratio can not be attained without further enlarging both the circular hole 54 and the rotary shaft 55. Even if such enlargements thereof were possible, it is would be either difficult or even impossible for an electrostrictive element having such a strain variation as 10 .mu.m to drive such an enlarged arrangement.
Furthermore, since both the radius R of the circular hole 54 and the radius r of the rotary shaft 55 can not be changed at will in the abovementioned prior art piezoelectric motor, the speed reduction ratio thereof has a fixed value, that cannot be changed as occasion demands.